Method and system for controlling an air-to-fuel ratio in a
non-stoichiometric power governed gaseous-fueled stationary internal
combustion engine

Abstract

A gaseous-fueled reciprocating internal combustion engine includes a
carburetor having a throttle valve that is controlled by a speed governor.
A proportional fuel control valve is disposed intermediate a fuel supply
and the carburetor, and is controlled by an air fuel computing device. The
computing device generates a control signal to adjust the fuel control
valve based on a governor sensed variable indicative of engine speed,
sensed engine torque, a governor output signal from the governor
indicative of an opening position of the throttle valve wherein 100%
corresponds to a wide open throttle position, and 0% corresponds to a
closed position, and a lean combustion control map containing
predetermined set point values stored in memory. During operation, the
control valve is responsive to the control signal generated by the
computing device for adjustment of a fuel flow therethrough so as to
obtain a ratio of air to fuel provided to the engine that is substantially
at a lean misfire limit of the engine, thereby reducing fuel consumption,
NO.sub.x emissions, and reducing exhaust gas temperatures. Alternatively,
the control signal is generated using engine speed alone.

a gaseous fueled stationary internal combustion engine having an air and
fuel delivery apparatus in communication with a plurality of combustion
chambers of said engine;

a governor configured to adjust said air and fuel delivery apparatus in
response to a first signal corresponding to a desired engine speed and a
second signal indicative of an actual engine speed, said governor being
further configured to output a third signal indicative of a governor
output position;

a control valve intermediate a supply of fuel and said air and fuel
delivery apparatus;

a computing device configured to generate an output signal for adjusting
said control valve according to a selected one of a first control strategy
and a second control strategy,

wherein in said first control strategy said governor is further configured
to output a third signal indicative of a governor output position, and
said computing device is configured to generate said output signal in
response to (i) said second signal; (ii) said third signal; (iii) a lean
combustion control map; and (iv) a torque signal indicative of engine
torque; and,

wherein in said second control strategy said computing device is configured
to generate said output signal in response to said second signal
indicative of engine speed alone;

wherein said control valve is responsive to said computing device output
signal for adjustment of a fuel flow therethrough so as to obtain a ratio
of air to fuel provided to the engine that is substantially at a lean
misfire limit of the engine.

2. The system of claim 1 wherein said selected one comprises said first
control strategy.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to a system and method of controlling an
internal combustion engine, and, in particular, to a system and method for
controlling an air-to-fuel ratio of a gaseous-fueled internal combustion
engine to a lean misfire limit of the engine.

2. Discussion of the Background Art

Owners and operators of industrial stationary engines have been concerned
with both the efficiency of operation (i.e., fuel consumption of such
engines) as well as emissions generated thereby for many years. In
particular, the owners and operators of industrial stationary engines are
subject to federal and state environmental regulations with respect to
combustion products of such engines, such as NO.sub.x, CO.sub.2, and other
emissions. Accordingly, there has been investigation into systems and
methods for controlling gaseous-fueled engines to reduce emissions of
certain types of combustion products. For example, conventional approaches
for reduction of, for example, NO.sub.x emissions, are obtained by
stoichiometric air-fuel (A/F) ratio operation in combination with
non-selective catalytic reduction technology (NSCR). However, this
approach uses a relatively increased amount of fuel.

Another approach taken in the art directed to optimizing efficiency and
emissions has been to operate such engines at air-fuel ratios lean of
stoichiometric. However, these approaches have shortcomings in producing
reliable and effective operation.

As background, it is a known characteristic of gaseous-fueled internal
combustion engines that they can be operated at air-fuel ratios lean of
stoichiometric. Operation at these "lean" air-fuel ratios may not produce
the output power called for; however, on the other hand, such operations
may occur at air-fuel ratios not lean enough to be at a lean misfire limit
of the engine. Thus, gaseous-fueled engines lose power (sometimes referred
to as a loss of reserve power capacity) when operated at "lean" air-fuel
ratios, even at air-fuel ratios substantially lean of stoichiometric,
before operating erratically.

One problem generally with reciprocating engines employing conventional
controls involves so-called "pumping losses." Known approaches for
adjusting the air and fuel delivery for gaseous-fueled engines have a
shortcoming in that even when operated at full rated power output, a
throttle valve in the air and fuel delivery apparatus (e.g., carburetor)
is not fully open. A undesirable trait of operating at less than wide-open
throttle (WOT) is increased "pumping losses" (i.e., horsepower wasted by
ingesting air through a flow limiting device, such as a partly closed
throttle valve). Ostensibly this failure to operate the engine at
wide-open throttle (WOT) is to allow the engine to have reserve power
capacity in the event of control system drift. Since control system drift
could not reliably be accounted for in known engine controls, it was
therefore necessary to operate with such a reserve capacity.

As a result of pumping losses, fuel consumption of the engine is increased,
thereby also increasing CO.sub.2 emissions. The increased amount of
combusted fuel elevates combustion temperatures, thereby increasing oxides
of nitrogen as a combustion product. In addition, combustion of the extra
fuel elevates temperatures, which in turn increases thermal stress on
various engine parts such as pistons, rings, valves, heads, exhaust
manifolds, etc. This increases maintenance costs.

There are primarily two control strategies for the "lean" control of the
air-fuel ratio of gaseous-fueled internal combustion engines: (i) open
loop control (i.e., with no feedback information), and (ii) closed loop
control (i.e., with feedback of a sensed variable indicative or otherwise
a measure of the combustion process itself in some way, such as the use of
an exhaust gas temperature parameter, an amount of oxygen in the exhaust
parameter, a fuel pressure parameter, etc.). These two control strategies,
as implemented in the art, have certain disadvantages.

Regarding known open loop air-fuel ratio control systems, a carburetor is
typically used as the air and fuel delivery apparatus. The carburetor, due
to the mechanics of the apparatus itself, fixes the ratio of air and fuel.
In the open loop approach, the system is adjusted to an air-fuel ratio
near the lean power loss/misfire limit. However, during operation, the
degree of optimization actually realized varies depending on a variety of
factors, such as changes in engine load, changes in relative humidity,
changes in fuel characteristics (e.g., BTU per SCF, flame speed, hydrogen
content, etc.), changes in atmospheric conditions, and the like. Inasmuch
as open loop control does not use any feedback, the degree of air-fuel
ratio "optimization" is left to the vagaries of system calibration drift,
mechanical mixing limitations of the carburetor itself, mechanical
degradation and changes in combustion variables such as ambient air
conditions and changes in fuel characteristics. Maintaining an acceptable
degree of air-fuel ratio optimization requires routine maintenance and
calibration, which can become costly and invasive. In addition, there are
reliability concerns. In particular, the engine can operate at air-fuel
ratios rich of the lean power loss/misfire limit, but cannot operate at
all at air-fuel ratios lean of the lean power loss/misfire limit.
Therefore, when variations, due to the above factors, occur tending to
lean the already predetermined "lean" air-fuel ratio provided to the
engine, drastic drop offs of power output may be observed, with operation
of the engine becoming erratic. In the worst case scenario, the engine may
stop operating all together. Inasmuch as this situation is commercially
unacceptable, the air-fuel ratio adjustment is configured so as to leave
the air-fuel ratio richer than an optimal "lean" air-fuel ratio by a
predetermined guard or safety margin. This safety margin is to allow for
the above-described degradation in air-fuel control that could result in
air-fuel ratios lean of the lean power loss/misfire limit being provided
to the engine. The disadvantage of including this guard or safety margin
is an increase in fuel consumption, which thereby directly increases
CO.sub.2 emissions, as well as elevates combustion temperatures (which
increases NO.sub.x. Known open loop control strategies have thus been
found unsatisfactory in the foregoing respects.

Known closed loop control strategies have similar disadvantages. In known
closed loop systems of the type including, for example, a carburetor, a
sensor (e.g., such as an exhaust oxygen sensor or an exhaust temperature
sensor) is used. The sensor provides a sensed variable signal that is
indicative of the combustion process. The sensed variable signal is used
in the control strategy to adjust the air-fuel ratio of the charge
provided to the engine. However, one disadvantage of such a system is that
the control of the air-fuel ratio can only be as accurate as the sensor
output itself. Second, while such a sensor does measure a
combustion-related event, it does not directly measure lean power
loss/misfire, per se. A third disadvantage involves the fact that this
approach is unable to detect (and thus track) factors such as ambient
atmospheric changes, changes in fuel characteristics or traits (e.g., BTU
per SCF, flame speed, hydrogen content, etc.), and sensor
degradation/drift. A fourth disadvantage involves the fact that such
sensor-based systems require regular calibration and maintenance checks,
which increases maintenance costs. A fifth disadvantage is that such
systems have an undesirable failure mode (i.e., sensors may fail in an
undesirable fashion, rendering the engine inoperative). Sixth, as the
engine itself changes with condition (e.g., wear), desired target values
change (to which the system is controlled using the sensor output) and
failure to make ongoing compensation to the predetermined "target" values
will cause the controlled air-fuel ratio to deviate from the programmed
optimum.

Therefore, to avoid reliability problems, such closed-loop systems are
operated at less than an optimal air-fuel ratio by including a guard or
safety margin. As noted above, including a "safety" margin generally
results in increased fuel consumption, increased Co.sub.2 emissions, as
well as elevated combustion temperatures (with the resulting undesirable
effects thereof noted above). Moreover, many of the known closed-loop
control systems employ a control action that is digital in nature (i.e.,
adjustments are made based on whether a sensor output is higher or lower
than a threshold value). This "dithering" has in many instances an
undesirable response characteristic.

Thus, there is a need to provide an improved system and method for
controlling a gaseous-fueled stationary internal combustion engine that
overcomes or minimizes one or more of the above-mentioned problems.

SUMMARY OF THE INVENTION

This invention provides for reliable, accurate control of an internal
combustion gaseous-fueled engine in a manner that simultaneously reduces
fuel consumption, oxides of nitrogen (NO.sub.x) emissions, and carbon
dioxide emissions (CO.sub.2). An engine controlled in accordance with this
invention uses approximately ten percent (10%) less fuel than conventional
stoichiometric controlled engines (i.e., an engine controlled for
stoichiometric operation equipped with a catalytic converter). Reduced
fuel consumption also results in about a ten percent (10%) decrease in
CO.sub.2 emissions. Furthermore, the present invention allows a lower
cooling burden than stoichiometric air-fuel engine operation of
approximately ten percent (10%) to fifteen percent (15%), thereby reducing
thermal stress on components in the combustion path with exhaust
temperatures reduced, in one embodiment, by approximately 125.degree. F.
The invention reduces overall fuel consumption, use of resources (e.g.,
fuel, cooling power, fired path engine parts last longer, etc.), while
broadly reducing both CO.sub.2, and NO.sub.x emissions, both gases of
which have a deleterious effect on the environment. Moreover, an engine
controlled in accordance with the present invention operates at a lean
power loss/misfire limit--an optimum air-fuel ratio. Preferably, an engine
operating in accordance with the present invention operates over a defined
engine operating envelope wherein emissions of NO.sub.x and CO.sub.2 are
known and can be monitored if needed by an emissions monitoring system.

In accordance with the present invention, a control system is provided for
a gaseous-fueled stationary internal combustion engine having an air and
fuel delivery apparatus in communication with combustion chambers of the
engine. The control system comprises a governor, a fuel control valve, and
a computing device. The governor is configured to adjust the air and fuel
delivery apparatus in response to a first signal corresponding to a
desired engine operating parameter and a second signal indicative of an
actual engine operating parameter. The governor is further configured to
output a third signal indicative of an opening position of the air and
fuel delivery apparatus. The fuel control valve is disposed intermediate a
supply of fuel and the air and fuel delivery apparatus. The computing
device is configured to generate a fourth signal, which is provided to the
control valve, for adjusting the valve. The fourth signal is computed in
response to (i) the third signal (i.e., governor output corresponding to
the opening position); (ii) a lean combustion control map; (iii) a fifth
signal indicative of engine speed; and, (iv) a sixth signal indicative of
engine torque. Adjustments of the control valve results in adjustments of
a fuel flow therethrough. In one embodiment, the fuel is adjusted (which
in turn causes the governor to impose a corresponding change in the
opening position of the air and fuel apparatus) so as to obtain a ratio of
air to fuel provided to the engine that is lean of a stoichiometric air to
fuel ratio of the engine.

In a preferred embodiment, the control valve adjusts fuel flow so as to
obtain a ratio of air to fuel that is substantially at a lean misfire
limit of the engine.

A primary advantage of the present invention is that over a broad range of
engine operation (at high rated power output), the engine is operated with
the air and fuel delivery apparatus at "wide open throttle" (WOT). Power
can be adjusted by control of the fuel amount by way of the control valve.
Operating the delivery apparatus at WOT reduces "pumping losses." In a
constructed embodiment, the lean combustion control map is populated with
predetermined data to achieve the above-mentioned WOT operation. In this
operating mode, the need for accurate sensing of any engine operating
measurements diminishes in importance.

When the power required of the engine is such that it cannot be operated
with the air and fuel delivery apparatus in a WOT state, operation at the
lean misfire continues to occur. In particular, the control established by
the present invention continues to operate the engine at the lean power
loss/misfire limit by controlling fuel independent of the air and fuel
delivery apparatus through control of the fuel control valve. Again,
preferably, the lean combustion control map is populated with parameter
values configured to achieve the foregoing result.

In another aspect of the present invention, a method of operating a
governed internal combustion gaseous-fueled engine is provided which
includes five basic steps. The first step involves determining a governor
sensed variable, preferably an engine speed parameter, and an engine
torque parameter. The next step involves selecting a prime governor output
set-point (PGOSP) parameter from a lean combustion control map using the
engine speed and engine torque parameters. The PGOSP parameter is
preferably expressed as a percentage of wide open throttle. Next,
determining a final governor output set-point (FGOSP) parameter using the
selected PGOSP parameter. The next step involves sensing a governor output
(GO%) parameter indicative of an opening position of an air and fuel
delivery apparatus for the engine. Preferably the GO% is expressed as a
percentage. In one embodiment where the air and fuel delivery apparatus
comprises a carburetor, a governor output parameter of 100% corresponds to
a wide open throttle (WOT) position of a carburetor throttle valve, while
a governor output parameter of zero percent (0%) corresponds to a fully
closed position. In one embodiment, the FGOSP parameter is a target for
optimal lean operation of the engine. Finally, the last step involves
decreasing fuel to the engine using a control valve intermediate a fuel
supply and the air and fuel delivery apparatus when the final governor
output set point (FGOSP parameter expressed as % of WOT) is less than the
actual governor output parameter (GO%). The decrease in fuel causes the
governor to increase the opening position of the air and fuel delivery
apparatus in order to maintain engine speed, thereby "leaning" the
mixture.

In a further embodiment, the method further includes the step of defining
values for the PGOSP parameters that populate the lean combustion control
map. The values are defined such that the control valve (under control of
the computing device) is controlled to adjust fuel flow therethrough so as
to obtain a ratio of air to fuel provided to the engine that is
substantially at a lean misfire limit of the engine.

In yet another embodiment, the step of decreasing fuel includes the substep
of generating a control signal to adjust the control valve using a
proportional-integral-derivative (PID) controller having at least two (2)
inputs: the FGOSP parameter and the GO% parameter. In a preferred
implementation, the PID controller is configured to have a slower response
than the control action of the governor.

Other objects, features and advantages will become clear or will be made
apparent during the course of the following description of a preferred and
other embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a combined block and diagrammatic view of a system in accordance
with the present invention for controlling the operation of a stationary
reciprocating gaseous-fueled internal combustion engine;

FIG. 2 is a diagrammatic view of an alternate, carburetor embodiment of the
air and fuel delivery apparatus illustrated in FIG. 1;

FIG. 3 is a simplified block diagram depicting, in further detail, the
computing device shown in FIG. 1;

FIG. 4 shows an exemplary lean combustion control map;

FIG. 5 shows an exemplary ignition control map;

FIG. 6 shows an exemplary biasing control map;

FIG. 7 is a simplified flow chart diagram depicting, in detail, the control
established by the embodiment shown in FIG. 1;

FIG. 8 is a simplified bar chart diagram illustrating a fuel consumption
reduction according to the present invention;

FIG. 9 is a simplified bar chart diagram illustrating a NO.sub.x emission
reduction according to the present invention;

FIG. 10 is a simplified bar chart diagram illustrating an exhaust gas
temperature reduction according to the present invention;

FIG. 11 is a combined block and diagrammatic view of an alternate
embodiment for controlling an engine according to the present invention;

FIG. 12 is a simplified flow chart diagram depicting the control
established by the embodiment of FIG. 11; and,

FIG. 13 is a simplified block diagram view of a still further embodiment
according to the invention featuring steering logic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before proceeding to a description of the invention referenced to in the
drawings, some basic definitions and a general overview of the control
established by the present invention will be set forth.

The term stoichiometric means a chemical reaction where there is neither an
excess nor a shortage of reactants. In hydrocarbon-fueled engines, this is
typically where there is neither an excess nor shortage of air to wholly
combust the fuel.

The term lean shall mean, in combustion, where there is more air to fuel
than stoichiometric.

The term rich shall mean, in combustion, where there is less air to fuel
than at stoichiometric.

The term misfire shall mean that point where an engine cylinder does not
fire consistently, often related to an air-fuel ratio of the charge being
combusted.

The term Engine Operating Envelope is an operating range of engine speeds
and engine torques, and may include ranges of temperatures, fuels, ambient
pressures, etc.

It is a general characteristic of gaseous-fueled internal combustion
engines that they can be operated at air-fuel ratios lean of
stoichiometric, not producing the power called for, but not yet lean
enough to be at the lean misfire limit. That is, gaseous-fueled engines
lose power (sometimes referred to as a loss of reserve power capacity)
when operated extremely lean of stoichiometric before operating
erratically. In addition, it is known that reciprocating internal
combustion engines expend and thus lose energy to ingest air for
combustion, cooling and scavenging purposes. This lost energy is commonly
referred to as "pumping losses." The more restricted an air inlet is, the
higher the pumping losses. For this reason, it would be desirable to
operate an engine with its air and fuel delivery apparatus as unrestricted
(i.e., open) as possible. In the art, however, the ability to operate with
an unrestricted ("wide open") air and fuel delivery apparatus is
compromised in that many such air and fuel delivery apparatuses depend on
a reduction in air pressure to ingest fuel, and then mix the fuel and air
together. Alternatively, other known air and fuel delivery apparatuses
cannot operate at the optimal point to minimize pumping losses without (i)
a multitude of sensors and (ii) a richer than optimal air to fuel ratio (a
safety margin) to ensure that the engine is not operated too lean (i.e.,
excessive misfire or stall).

The control system in accordance with the present invention improves fuel
efficiency by reducing pumping losses of the engine. Pumping losses are
reduced by operating the engine at wide open throttle (WOT), or as wide
open as possible for any particular power setting. A reduction in pumping
losses has the direct effect of reducing the total amount of fuel
consumed, which in turn reduces emissions of CO.sub.2 in proportion to
fuel consumption (assuming a carbon-containing fuel is used). The
reduction in fuel consumption is also operative to reduce heat rejected to
the heat cooling system, which reduces horsepower required to cool the
engine, or, releasing cooling constrained horsepower. In addition,
reducing fuel consumption also reduces the thermal stress on engine parts
in the fired gas path (e.g., valves, pistons, rings, heads, etc.).

In addition, the invention also provides the benefit of an increase in mass
of the cylinder charge for a fixed amount of fuel. Increasing the mass has
a direct effect in reducing the formation of oxides of nitrogen
(NO.sub.x), an Environmental Protection Agency (EPA) criteria air
pollutant. A reduction is observed because the formation of NO.sub.x is
primarily a function of time and temperature. The formation of NO.sub.x
versus temperature increases logarithmically. Thus, by increasing the
amount of air for a fixed amount of fuel, the combustion temperature is
lowered, because there is a larger mass available to absorb the heat of
combustion. Since the formation of NO.sub.x is logarithmic, even a modest
reduction in combustion temperature has a dramatic effect on the formation
of NO.sub.x. Thus, since a control system in accordance with the present
invention maximizes the amount of air into the combustion chamber, it has
the positive effect of reducing NO.sub.x. Moreover, since the amount of
air is maximized, the combustion chamber is cooled by the added
scavenging. Moreover, post-combustion free radicals are better purged from
the cylinder. In addition, since combustion is operating at a very lean
air-fuel ratio, the combustion process is more detonation resistant due to
the lower flame speed and cooler components (exhaust valves in
particular).

Referring now to the Figures wherein like reference numerals are used to
identify identical components in the various views, FIG. 1 shows a control
system 10 in accordance with the present invention for controlling an
engine 12. In a preferred embodiment, engine 12 comprises a gaseous-fueled
stationary reciprocating internal combustion engine 12. Engine 12 is not a
relatively small displacement engine of the type used in self-propelled
vehicles such as automobiles, but rather, is a relatively large
displacement, stationary engine. Such engines are adapted, in a preferred
environment, for use at a compressor station to provide sufficient fluid
power to ensure the proper progress of a transported fluid, for example,
natural gas, through a pipeline (not shown). Such an environment is known,
as described and illustrated in U.S. Pat. No. 5,703,777 to Buchhop et al.,
entitled "Parametric Emissions Monitoring System Having Operating
Condition Deviation Feedback," owned by the common assignee of the present
invention, and herein incorporated by reference in its entirety. Each
compressor station may include plurality of engines 12. Moreover, it
should be understood that there may be a plurality of compressor stations
along a section of the pipeline. Engine 12 may be a reciprocating type
engine comprising conventional and well-known components such as is
available from, for example, Ingersoll-Rand as either a Small V-type Gas
(SVG) engine, or a King-size V-type Gas (KVG) engine. It should be
understood, however, that the present invention may be usefully applied in
applications, such as where engine 12 drives electric generators,
irrigation pumps, and the like. In addition, the present invention may be
usefully applied to engine 12s having a broad range of displacements.

Engine 12 comprises an air and fuel delivery apparatus 14 having one or
more throttle valves 15, and which in a constructed embodiment comprises a
carburetor 14' having a throttle valve 15 (carburetor 14' best shown in
FIG. 2). Engine 12 further comprises an exhaust manifold 16, an output
shaft 18, a flywheel 20, and a plurality of combustion chambers (not
shown).

The air and fuel delivery apparatus 14 is in communication with the
combustion chambers of engine 12 and has an air inlet coupled to an air
supply (not shown), and has a fuel inlet configured to receive fuel
originating from a fuel supply 22. Apparatus 14 (or 14') is adjusted to
control the air and fuel delivered to the combustion chambers of engine
12. Apparatus 14 (or 14') may also comprise conventional and known
apparatus such as a mixer, fuel injector(s) and the like.

FIG. 1 also shows control system 10. Control system 10 according to the
invention includes a governor 24, a fuel control valve 26, an air fuel
computing device 28, a means or circuit 30 for determining a governor
sensed variable preferably engine speed, a means or circuit 32 for
determining engine torque, and a means or circuit 34 for determining a
plurality biasing parameter values.

Governor 24 is configured to adjust air and fuel delivery apparatus 14 in
response to a first signal (designated S.sub.1 in the drawings)
corresponding to a desired engine operating parameter, and further in
response to a second signal (designated S.sub.2) indicative of an actual
engine operating parameter. Herein the second signal S.sub.2 comprises a
governor sensed variable (GSV) and will be referred to as the governor
sensed variable S.sub.2 or simply GSV. Also, the first signal S.sub.1,
comprises a governor set point (GSP) parameter and will be referred to as
the governor set point signal S.sub.1 or simply GSP.

In a constructed embodiment, the GSP parameter corresponds to a desired
engine speed (e.g., target engine RPM) while the GSV parameter corresponds
to an actual engine speed (i.e., actual engine RPM). In one configuration,
an operator of engine 12 inputs a desired engine speed, as well as a
desired engine torque (which may occur in stepwise increments but which
may also occur in a stepless analog fashion) to define a power output for
engine 12. Governor 24 is further configured to output a third signal
(designated S.sub.3) indicative of a governor output position.

The governor output position corresponds to an opening position of air and
fuel delivery apparatus 14 inasmuch as the two are mechanically coupled.
In the embodiment wherein engine 12 includes carburetor 14' (FIG. 2)
having throttle valve 15 associated therewith, third signal S.sub.3
comprises a governor output (GO%) parameter indicative of the relative
opening of throttle valve 15 (expressed as a percentage of wide open
throttle). A 100% value for GO% corresponds to a wide open throttle (WOT)
position for throttle valve 15 and a 0% value for GO% corresponds to a
fully closed position for throttle valve 15.

It should be understood by those of ordinary skill in the art that for most
engines 12, the GSV parameter comprises sensed engine speed; however the
GSV parameter may be one selected from the group consisting of a sensed
engine speed, a sensed engine ignition events per unit time, and a sensed
air and fuel delivery apparatus air vane force, among others. Likewise,
the GSP parameter will be described in a preferred embodiment as
comprising a predetermined engine speed; however, it should be understood
that the GSP parameter may be one selected from the group consisting of a
predetermined engine speed, a predetermined engine ignition events per
unit time, and a predetermined air and fuel delivery apparatus air vane
force, so as to match up and correspond with the selected GSV parameter.
Governor 24 may comprise conventional and well-known apparatus.

Governor 24 may itself be a stand-alone device, or may be implemented
(e.g., through software) in computing device 28. It is shown as a separate
block in the Figures for clarity only to emphasize the independent
function it performs. Governor 24 may control air and fuel delivery
apparatus 14 in accordance with known control approaches, such as via a
Proportional-Integral-Derivative (PID) control approach. Its response
characteristics can therefore be programmed or otherwise configured to
attain the desired response.

Fuel control valve 26 is disposed intermediate fuel supply 22 and air and
fuel delivery apparatus 14. In a preferred embodiment, fuel control valve
comprises a proportional control valve 26 which includes an
electromagnetic portion such as an actuator 36, and a regulating plate or
the like 38 operating in unison therewith to adjust the flow of fuel
through the valve. Proportional control valve 26 may comprise conventional
and known components.

Computing device 28 is configured to generate a fourth signal (designated
S.sub.4) for adjusting valve 26 in response to the following signals and
in a manner to be described hereinafter: (i) the GO% parameter; (ii) a
lean combustion control map, such as map 40 (best shown in FIG. 4); (iii)
a fifth signal (designated S.sub.5) indicative of engine speed from engine
speed determining means 30; and, (iv) a sixth signal (designated S.sub.6)
indicative of engine torque generated by engine torque determining means
32. Control valve 26 is responsive to fourth signal S.sub.4 for adjustment
of a fuel flow therethrough so as to obtain a ratio of air to fuel
provided to engine 12 that is lean of a stoichiometric air to fuel ratio
of engine 12, or, preferably, at a lean power loss/lean misfire limit.

Computing device 28 includes central processing means (not shown), Random
Access Memory (RAM) (not shown), Read-Only Memory (ROM) (not shown), and
an Input/Output (I/O) interface (not shown). Computing device 28 is
configured to store predetermined data, for example in RAM or ROM, for
operating and control of engine 12. Computing device 28, in accordance
with known practice, can perform electronic signal processing involving
logic and programmed computations. For purposes of example only, computing
device 28 may be programmed to control certain functions pertaining to the
operation of engine 12 not directly related to air and/or fuel, such as
ignition timing.

FIG. 5 shows ignition table 42 having timing entry values 44 (preferably in
degrees relative to top dead center (TDC), positive values being degrees
before TDC). These timing values 44 may be retrieved and used based on the
actual engine speed and engine torque as input parameters.

With continued reference to FIG. 1, computing device 28 may comprise
conventional and known apparatus, such as a 3300 series device, in
particular model No. 3350, commercially available from the Bristol Babcock
Company, which is an analog-controlled digital computer. It should be
understood that there are a plurality of alternative computing devices
commercially available and known to those of ordinary skill in the art
suitable for use in practicing the present invention.

FIG. 1 further shows means 30 for determining the GSV parameter, which
preferably includes means for determining an engine speed (expressed in
RPM) parameter. It should be understood from the above-described
alternatives for the GSV parameter that corresponding alternative means or
circuits may be suitably employed. In a preferred embodiment, where the
GSV parameter comprises engine speed, means 30 may include conventional
and known engine RPM sensors, which generate an output signal indicative
of the sensed engine speed.

Engine torque determining means 32 is configured to generate sixth signal
S.sub.6 indicative of the torque being produced by engine 12. Means 32 may
comprise conventional and known torque sensing components for directly
measuring engine torque. In an alternative embodiment, means 32 may
comprise means for indirectly measuring engine torque, such as sensor
outputs and programming to implement a so-called fuel torque method using
engine speed and fuel flow parameters as inputs, as described in U.S. Pat.
No. 5,703,777 entitled "Parametric Emissions Monitoring System Having
Operating Condition Deviation Feedback", assigned to the common assignee
of the present invention. Further alternative approaches may be employed,
as known to those of ordinary skill in the art.

FIG. 3 shows computing device 28 in greater detail. As described above,
computing device 28 may be configured by way of programming to perform a
variety of logic functions and predetermined computations. For example,
Appendix A hereto shows programming code used in a constructed embodiment,
such code being written as a programming language referred to as the ACCOL
programming language established by the Bristol Babcock Company. Computing
device 28, in accordance with the present invention, includes data
comprising lean combustion control map 40, including a plurality of prime
governor output set-point (PGOSP) parameters 46 (best shown in FIG. 4, and
expressed as a %). Computing device further includes means or circuit 48
for selecting one of the plurality of PGOSP parameters as a function of
engine speed and engine torque, means or circuit 50 for determining a
final governor output set-point (FGOSP) parameter (expressed as a %) which
may optionally include means or circuit 52 for biasing the selected PGOSP
parameter, means or circuit 54 for determining a difference between the
governor output signal S.sub.3 (expressed as a %) and the FGOSP parameter
(also as a %), and means or circuit 56 for generating fuel valve control
signal S.sub.4, which may include a proportional-integral-derivative (PID)
controller 58 configured to generate fourth signal S.sub.4.

In one embodiment, the PGOSP parameters 46 are selected such that valve 26
is controlled (according to fourth control signal S.sub.4) to adjust fuel
flow therethrough so as to obtain a ratio of air to fuel that is lean of a
stoichiometric air to fuel ratio of engine 12. In a preferred embodiment,
however, the PGOSP parameters 46 are configured so as to obtain a ratio of
air to fuel provided to engine 12 that is substantially at a lean misfire
limit of engine 12. As shown in FIG. 4, the PGOSP parameter values
correspond to an air and fuel delivery apparatus opening position
expressed as a percentage of wide open throttle.

The method for determining values for PGOSP parameters 46 to populate
control map 40 can be accomplished in a selected one of at least two ways:
(i) using empirical data from tests; and (ii) utilization of mechanical
and thermodynamic models.

Regarding the empirical data approach, it is generally necessary to make
engine performance measurements for each engine type to be controlled in
accordance with the invention. To populate map 40, an engine similar in
type to the selected engine (i.e., similar to engine 12) is operated over
its user defined engine operating envelope (EOE). It should be understood
that both the range of map 40 (i.e., lowest to highest engine speed
torque), as well as its resolution (i.e., increment or delta value), may
be selected in accordance with the required performance. The range and
resolution of the map 40 shown in FIG. 4 was found satisfactory for an
actual Ingersoll-Rand brand SVG engine. Acquisition of the empirical data
can be commenced by operating the engine at full rated power (highest
engine torque/speed product). It is well known that the product of engine
torque and engine speed is engine Brake horsepower (BHP). This starting
point has the benefit of guiding the user as to the range of the map if
not already predetermined (e.g., as emission reduction goals and, fuel
reduction goals are satisfied, acquisition of further data points may be
discontinued). In a preferred embodiment, lean combustion control map 40
is defined as a function of engine torque and engine speed (at a given
ambient temperature). As shown in FIG. 4, across the top row is listed the
engine torque as a percentage of the maximum rated torque of the engine.
Down the first column of FIG. 4 is listed the GSV parameter which, in the
map is expressed as a percentage of the maximum rated speed of the engine.

To populate map 40 for a specific speed/torque combination, the engine is
operated at that combination according to normal conventional control. For
example, the throttle valve may be set to 50% of WOT. Then, the user
incrementally reduces fuel to the air and fuel delivery apparatus 14. This
fuel reduction may be accomplished in a data acquisition mode by employing
a fuel control valve such as valve 26. This fuel reduction in turn reduces
the operating speed of the engine. The governor 24 responds by "opening
up" air and fuel delivery apparatus 14 further in order to maintain the
set engine speed (i.e., the GSP parameter). In this example, the throttle
valve may open up to 60%, then 70%, then 80%, and so on. This iterative
process (decrease fuel, observe throttle open up) continues until either
(i) the governor output is 100% (WOT) (i.e., further decreases of fuel
will not result in further "opening up" of the throttle valve) or (ii) a
lesser setting is found. The governor output parameter (GO%) is then
recorded for that engine speed and engine torque. Regarding what
constitutes a suitable "lesser setting" (i.e., less than 100% of WOT), the
primary factor is engine performance. That is, the PGOSP shall be the
governor output beyond which further "leaning" results in an unacceptable
amount of engine speed variation (i.e., RPM instability), a reduction in
power below a desired amount or unacceptable engine misfire. Other
performance parameters, such as NO.sub.x emissions, fuel consumption,
exhaust gas temperature (EGT), thermal stress and cooling system burden,
and the like may also be monitored and used to inform the selection of
what governor output is recorded. This overall process is repeated for
each PGOSP parameter required in map 40.

Regarding utilization of mechanical and thermodynamic models to populate
lean combustion map 40, the amount of air for optimal engine operation
(i.e., combustion) may be determined using known models. In addition, by
knowing the flow characteristics (C,) of the air flow controlling valve
(e.g., throttle valve 15) of air and fuel delivery apparatus 14, a
throttle valve position can be derived to deliver the predetermined amount
of air. This can be thereafter converted to a percentage of wide open
throttle for populating map 40.

With continued reference to FIG. 3, PGOSP parameter selecting means 48 may
comprise well-known routines for retrieving data (e.g., a PGOSP parameter
46) from a two-dimensional matrix (e.g., map 40) as a function of two
input variables (e.g., engine speed indicative signal S.sub.5 ; and,
engine torque indicative signal S.sub.6) FGOSP parameter determining means
50 is implemented in a selected one of at least two ways. In a first
implementation, the selected PGOSP parameter (i.e., retrieved from map 40
by selecting means 48) directly defines the final governor output
set-point (FGOSP) parameter. In a second implementation, however, biasing
means or circuit 52 is used to modify the selected PGOSP parameter to
yield the FGOSP parameter. A variety of process (e.g., load on a
pipeline), engine, and environmental parameters may affect, in some
degree, the amount of required air for optimal lean combustion. For
example, since the temperature of the intake air in the intake manifold
affects the density of the combustion air, it has an effect on the
air-fuel ratio. This environmental change thus affects how the PGOSP
parameter selected from lean combustion control map 40 controls the
operation of engine 12. Accordingly, to account for changes in the air
manifold temperature (AMT), a further control map may be provided, in an
alternate embodiment. In particular, an AMT control map or table 54 (best
shown in FIG. 6) is provided, and defined, for example, using empirical
data, the ideal gas law and other thermodynamic data, or a combination(s)
thereof. Table 54 contains a multiplicative factor that is used by means
52 to bias the selected PGOSP parameter 46 to arrive at a biased PGOSP
parameter (designated BPGOSP). The appropriate bias factor to retrieve
from Table 54 is based on the sensed AMT, which may be determined from one
of signals S.sub.7 -S.sub.n.

In addition, biasing means 52 may include further maps for improving engine
operation. Any engine operating parameter that affects engine performance,
but has a consistent effect on such performance, may be, but need not be
mapped and used. For example, ignition timing affects engine performance
but need not be mapped, assuming it consistently controls engine operation
in a known and repeatable fashion.

For any particular engine, should the assumption of consistent affect on
engine performance not be true, then a bias map (containing, for example,
multiplicative factors) may be implemented to improve engine performance.
For example, for a particular engine wherein the ignition timing has a
variable effect on engine performance, then an ignition bias map,
containing a multiplicative factor, may be generated. The factors can then
be used to adjust the PGOSP parameter values 46 from map 40 to produce a
biased PGOSP parameter. This biasing technique can be used with any number
of parameters such as relative humidity, barometric pressure, etc., as
required for a particular engine type. In the empirical approach, all of
the respective bias factors are retrieved from the respective maps or
tables using one or more of the biasing signals S.sub.7 -S.sub.n perhaps
along with the GSV parameter and engine torque, as indices, and
thereafter, all the bias factors are applied in a multiplicative fashion
to arrive at a biased PGOSP (BPGOSP) parameter.

It should be understood that the factors in Table 54 in particular, and a
biasing factor for any selected operating parameter in general, need not
be "multiplicative" in nature. The biasing may occur through evaluation of
a function (or a mathematical function), through an additive or
subtractive factor, a dividing factor, evaluation of logarithm, taking of
a derivative, exponential, etc., or a combination thereof the manner in
which the selected operating parameter changes the optimal air-fuel ratio
determines the actual relationship used for biasing.

Determining means 50 of computing device 28 is programmed to select either
the PGOSP parameter or the biased PGOSP parameter, which then becomes the
final governor output set-point (FGOSP) parameter (which is expressed as
%).

It should be understood that known programming techniques exist to retrieve
data from various biasing tables in accord with the run-time biasing
parameter values (S.sub.7 -S.sub.n) as indexes.

With continued reference to FIG. 3, governor output/GFOSP difference
determining means 54 may be implemented using arithmetic logic functions
available generally in device 28. This "difference" may be viewed as an
"error" signal in control theory parlance. The "difference" signal is
between the actual throttle plate 15 opening position (GO%) and the
desired, target throttle plate opening position (FGOSP) for optimal lean
operation.

Fuel valve control signal S.sub.4 generating means 56 preferably includes
PID controller 58. PID controller 58 is a well understood control block in
the field of control theory for generating an output signal (in this case,
S.sub.4) in a way so as to reduce an "error" signal (e.g., the
above-described difference between the GO% and FGOSP parameters). A PID
control may be implemented by exercise of known programming practices by
one of ordinary skill in the art. For example, if governor output
parameter GO% is less than the target (e.g., the FGOSP parameter), then
generating means 56 via PID controller 58 adjusts signal S.sub.4 so as to
decrease fuel flow through valve 26. This causes governor 24 (and its
control mechanism) to "open up" or, in other words, increase the actual
air and fuel delivery apparatus 14 opening position, which will be
reflected in an increased GO% value. This increased GO% reduces the
"difference" or "error" signal, as desired. Generating means 56 makes no
modifications to signal S.sub.4 when the GO% parameter is equal to the
FGOSP parameter. Generating means 56 will vary signal S.sub.4 when the GO%
parameter is greater than the FGOSP parameter so as to increase fuel flow
through valve 26, whereby the governor 24 "closes down" throttle plate 15
of apparatus 14 (GO% decreases).

Referring now to FIG. 7, an overall description of a preferred embodiment
of control system 10 will be set forth.

Control system 10 requires that engine 12 be governed in some way (e.g.,
engine speed governed), and that the default state when no control is
imposed is always at least slightly lean of stoichiometric. In step 70,
engine 12 is operating, governed, and producing useful power (i.e., not at
idle). In particular, "governed" in the preceding sentence means that
governor 24 maintains the GSV parameter substantially equal to the GSP
parameter. In the preferred embodiment, governor 24 is a speed governor.
Further, in a preferred environment, an operator selects a desired engine
speed, and the governor operates to maintain the actual speed (GSV) equal
to the selected speed (equal to the GSP). Moreover, the user selects a
desired engine torque (which may occur in a stepwise fashion, as a percent
of rated output torque but which may also occur in a stepless analog
fashion).

In step 72, actual engine torque, as sensed by engine torque determining
means 32, is determined and is provided to computing device 28 by way of
sixth signal S.sub.6. In step 74, in the preferred embodiment, engine
speed is measured by engine speed determining means 30 and is provided as
signal S.sub.5 to computing device 28.

In step 76, a prime governor output set point (PGOSP) parameter is
determined by computing device 28, using sensed engine torque and sensed
engine speed as input indices into lean combustion control map 40. Step 76
is an important step inasmuch as it determines the correct control point
to operate engine 12 to achieve an air-fuel ratio provided to the engine
at a lean power loss/misfire limit, the engine's most fuel efficient
point, as well as the point producing the minimum amount of NO.sub.x and
CO.sub.2.

Steps 78 and 80 are performed only in yet another alternate embodiment
wherein biasing is used. Step 78 involves determining one or more biasing
operating parameters using biasing parameter determining means 34
corresponding to a number of various engine operating parameters (OP).
These values or signals are provided to computing device 28 as signals
S.sub.7 -S.sub.n.

In step 80, a biased prime governor output set point (BPGOSP) is determined
by computing device 28, using biasing parameter signals developed in step
78, in a manner described in detail above (e.g., the multiplicative
biasing factors found in table 54 of FIG. 6).

In step 82, computing device 28 determines whether to utilize the PGOSP
parameter from step 76, or the BPGOSP parameter from step 80, and
thereafter generates the final governor output set point (FGOSP)
accordingly.

Steps 84, 86 and 88 are performed in still yet another alternate
embodiment, and which provides computing device 28 with information
regarding the state of operation of engine 12. In particular, step 84
involves measuring the governor sensed variable (GSV), namely engine speed
in one embodiment. Step 86 involves identifying the governor set point
(GSP) parameter. In step 88, computing device 28 determines whether the
GSV parameter is equal to the GSP parameter. These steps (84, 86 and 88)
can be utilized and are utilized in this alternate embodiment by computing
device 28 to halt further control actions according to the invention, or
to alter the rate thereof. In particular, steps 84, 86 and 88 may be
profitably employed in situations where disturbances to a steady state
operation of the engine may be expected, for example, where large power
demand changes have been made on the engine, during start, warm-up, load
pick up, load shed, cool down, and stop. It should be appreciated that
steps 84, 86 and 88 collectively provide computing device 28 with
knowledge of the engine's ability and readiness to accept control
established by the present invention. Note that steps 84, 86 and 88 are in
the nature of a one shot process. That is, steps 84, 86 and 88
collectively define an optional enabling step, and once completed, need
not be repeated during the remainder of the engine operation (except for
the above-identified instances where steady state operation may not be
attained or where steady state may be disturbed).

In step 92, computing device 28, more particularly PID controller 58,
compares the governor output (GO%), which corresponds to a percent of WOT
of the throttle valve, to the final governor output set point (FGOSP),
which is the target percent of WOT. If GO% is equal to FGOSP, then no
change is required and the control is returned to step 76. Engine 12 is at
governor equilibrium and at the lean power loss/misfire limit thereof.
Both governor 24, and PID control 58 are at equilibrium (and have thus
satisfied the predetermined respective set points).

Otherwise, if GO% and FGOSP are unequal, control is passed to step 94. In
step 94, PID control 58 determines whether the governor output (GO%) is
greater than the FGOSP parameter. If so, the process progresses to step
96, wherein signal S.sub.4 is adjusted proportionately to increase fuel
through valve 26 (and thus to engine 12). The increase in fuel flow
accomplished by step 96 enriches the air-fuel ratio and results in an
increase in the governor sensed variable GSV (e.g., engine speed in a
preferred embodiment). The increase in engine speed causes governor 24 to
"close down" air and fuel delivery apparatus 14 so as to bring the engine
speed to the value of the governor set point (GSP) which is in RPM.
Accordingly, the GO% parameter, which is indicative of the throttle valve
opening position, increases.

Otherwise the process goes to step 98, which calls for a decrease in fuel
flow by adjusting control valve 26. It should be understood that step 96
is repeated in predetermined increments until governor 24 is at
equilibrium, with the governor set point (GSP) satisfied (i.e., GSV=GSP),
and the final governor output set point FGOSP (calculated by computing
device 28) satisfied (i.e., GO% =FGOSP). System 10 thus is at both
governor equilibrium, as well as PID controller 58 equilibrium (i.e.,
operating with air-fuel ratios at the lean power loss/misfire limit of
engine 12). The foregoing also applies to step 98.

Another advantage of the invention is the desirable failure mode.
Preferably, proportional fuel control valve 26 is selected to fail in an
unrestricted or open state (i.e., "on shelf condition"). Thus, should
computing device 28 fail, then the fuel control is designed to fail fully
open on a loss of signal. Engine 12 is still governed in a conventional
manner, only without the benefit of the air-fuel ratio control established
by the present invention. Engine 12 is left operable, and an operator can
be notified by an alarm in such an event or the engine may be stopped by
the alarm automatically at the option of the operator.

Another advantage of control system 10 is that inasmuch as the engine
operating envelope must be defined (e.g., both the speed and torque of the
engine are known) for the control to be imposed, the emissions from the
engine may be determined, for example, via the methodology described and
claimed in U.S. Pat. No. 5,703,777 entitled "Parametric Emissions
Monitoring System Having Operating Condition Deviation Feedback." This
provides benefits to users of such engines who must know emissions for
record keeping or to ensure emission permit compliance. If the engine
health degrades appreciably, operation may become unstable, which may in
turn cause an alert to be asserted whereby the operator or control system
may take remedial action.

While the foregoing description makes reference to the PID controller's 58
interaction to (or iterative reaction with) governor 24, this invention
does not require active governor control in all engine operating modes. At
high engine power outputs, for example, governor 24 may be driven to 100%
output (WOT). In such a mode, the power control of engine 12 is "governed"
by the PID controller's 58 regulation of the fuel circuit alone (which is
in turn driven by the FGOSP parameter value). This control action
transition to and from governor 100% output is seamless, continuous,
analog, proportional, bumpless and accomplished without two state (on/off)
sensors and "dithering" control action.

A method in accordance with the present invention, described in the
foregoing paragraphs, is repeated continuously in an infinitely
proportional analog process to maintain engine air-fuel mixture optimized
at all times.

FIG. 11 shows an alternate control system embodiment, designated 10'. While
control system 10 shown in FIG. 1 may be characterized as a two-input
system (governor sensed variable GSV and engine torque), since system 10
uses these parameters as primary control inputs, system 10' shown in FIG.
11 may be referred to as a single input system (governor sensed variable
only).

Control system 10' is similar in configuration to control system 10, except
that computing device 28 requires only the GSV parameter to implement its
control. Only the differences between system 10 and 10' will be described,
it being understood that the foregoing description and illustrations of
system 10 apply here to system 10' with equal force unless specifically
noted to the contrary. Engine torque determining means 32 has been
eliminated in control system 10', and has been replaced by block 100
defining a group of timers and engine permissives configured to assure
that engine 12 is in a steady state mode (ready to be controlled by
control system 10'). Block 100 is not required by control system 10.

FIG. 12 shows a simplified flow chart diagram illustrating the control
established by control system 10'. The method illustrated shows a control
for an engine 12 such that an air-fuel ratio provided to the engine is
lean of stoichiometric, and, preferably, substantially at the lean power
loss/misfire limit, the same as control system 10.

In step 102, engine 12 is operating, is governed, and is producing useful
power (i.e., the engine is not at idle). In particular, "governed" here
means governor 24 is maintaining the GSV parameter substantially equal to
the GSP parameter.

In step 104, computing device 28 receives as an input a governor sensed
variable (GSV) from determining means 30, which preferably is engine speed
(RPM). This step performs the function of obtaining information pertaining
to an actual engine operating parameter.

In step 106, computing device 28 determines a parameter pertaining to a
desired or target engine operating parameter. The units will correspond to
whatever parameter is sensed in step 104. Preferably, in step 106, device
28 calculates an air-fuel computing device controller set point (AFCDCSP)
based on a governor set point (GSP) parameter and a k factor such that:

AFCDCSP=(GSP-k)

The AFCDCSP is also preferably in units of speed (RPM).

The value of k is determined using either (i) data determined empirically
for engine 12, or (ii) from combustion models, thermodynamics and
scientific methodology, or a combination of the two. Step 106 is at the
core of the operation of this alternate embodiment of the invention,
control system 10'. Step 106 determines the correct control point to
operate engine 12 such that operation occurs substantially at the lean
power loss/misfire limit --engine 12's most fuel efficient point, as well
as the point producing the minimum amount of NO.sub.x and CO.sub.2. The
set point of governor 24 (GSP) can be, but need not be, determined by the
computing device 28. As would be understood by one of ordinary skill in
the art, governor 24 may be a stand-alone device, or, may be embedded in
the programmed logic of computing device 28. For this reason and for
clarity, the entire governor control scheme per se, is not shown in
Figures. It should be understood that air-fuel computing device 28 (AFCD)
may include, and preferably includes, a PID controller or equivalent
controller, shown in FIG. 11 as PID controller 58'. Steps 108 and 110 are
performed in an alternate biasing embodiment of control system 10'. In
step 108, a number of engine operating parameters (OP) are sensed, and, in
step 110, utilized by computing device 28 to modify the air-fuel computing
device controller set point (AFCDCSP). The methodologies and
implementations described above in connection with biasing apply equally
to control system 10'. As with system 10, the biasing parameter sensors
for system 10' may include an ambient temperature sensor, an air manifold
pressure sensor, an exhaust gas temperature sensor, an exhaust gas
pressure sensor, an engine torque sensor, an exhaust gas oxygen sensor, an
exhaust gas NO.sub.x sensor, a relative humidity sensor, a barometric
pressure sensor, a cylinder firing pressure sensor, and an ignition angle
sensor. These sensors generate a plurality of sensed parameter indicative
signals S.sub.7 -S.sub.n, which are provided to computing device 28 in
order to bias the target engine speed AFCDCSP to yield a modified or
biased set point, designated the BAFCDCSP parameter.

In step 112, computing device 28 determines whether to utilize either the
BAFCDCSP (biased) set point parameter or the AFCDCSP (base line) set point
parameter, and thereafter generates a final AFCDCSP designated (FAFCDCSP)
parameter. This is also shown diagrammatically in FIG. 11, where the final
controller set point (RPM) is provided to PID controller 58.

Steps 114, 116, 118, and 120 are performed in a further alternative
embodiment of control system 10'. These steps are adapted to provide
computing device 28 with information pertaining to the engine 12's ability
and readiness to submit to the control established by system 10'. These
steps (114, 116, 118 and 120) can be utilized and are utilized in this
alternate embodiment by computing device 28 to halt further control
actions according to the invention, or to alter the rate thereof. In
particular, steps 114, 116, 118 and 120 may be profitably employed in
situations where disturbances to a steady state operation of the engine
may be expected, for example, where large power demand changes have been
made on the engine, during start, warm-up, load pick up, load shed, cool
down, and stop. It should be appreciated that steps 114, 116, 118 and 120
collectively provide computing device 28 with knowledge of the engine's
ability and readiness to accept control established by the present
invention. Note that steps 114, 116, 118 and 120 are in the nature of a
one shot process. That is, steps 114, 116, 118 and 120 collectively define
an optional enabling step, and once completed, need not be repeated during
the remainder of the engine operation (except for the above-identified
instances where steady state operation may not be attained or where steady
state may be disturbed). For embodiments of control system 10' that do not
utilize steps 114-120, the control action and rate may nonetheless be
adjusted by proper selection of constants for PID controller 58' such that
it does not cause conflict with control imposed by governor 24.

In step 122, computing device 28, and in particular PID controller 58',
receives as an input the governor sensed variable GSV, which is preferably
engine speed (expressed in RPM). In addition, PID controller 58' is also
provided the final controller set point (expressed in RPM) designated
FAFCDCSP (step 112). When the GSV parameter is equal to the final
controller set point FAFCDSP (expressed in RPM), no control action is
taken and the process is returned to step 106.

Otherwise, the process progresses to step 124. In step 124, if the governor
sensed variable GSV (e.g., engine speed expressed in RPM) is below the
desired, final controller set point FAFCDSP (also expressed in RPM),
controller 58' increases fuel flow by adjusting fourth signal S.sub.4
which is provided to fuel control valve 26. This increase is shown in step
126. Increasing fuel flow enriches the air fuel mixture to engine 12,
which has the effect of increasing the actual engine speed, which will
thereafter be picked up by PID controller 58' by way of the governor
sensed variable GSV provided thereto.

While step 126 is configured to satisfy the PID controller 58' set point,
governor 24 imposes its own control scheme. In particular, governor 24
senses the governor sensed variable GSV (engine speed) and compares it to
the governor set point GSP signal S.sub.1. Governor 24 thereafter adjusts
its output to proportionally drive the air and fuel delivery apparatus 14
such that a new equilibrium point is achieved, wherein the governor set
point GSP is satisfied. Here, since the engine speed has increased, the
governor 24 will now take action based on the present GSP parameter, and
either "open-up" or "close down" throttle valve 15.

At equilibrium, neither the PID controller set point (FAFCDCSP expressed in
RPM), nor the governor set point (GSP also expressed in RPM) are
simultaneously and mutually satisfied. Instead, PID controller 58' and
governor 24 (also a controller) work in opposite directions, maintaining
engine 12 in a "state of tension." The parameter "k" establishes the gap
between the two set points. At this time, engine 12 is in an instantaneous
equilibrium state, at a lean power loss/misfire limit of engine 12 wherein
fuel consumption, formation of NO.sub.x, formation of CO.sub.2, and heat
rejected to the cooling system are minimized, and engine 12 is very nearly
at its desired governor set point GSP (which may be selected by an
operator of engine 12, who may reasonably expect engine 12 to operate
substantially at its set speed). The above-described equilibrium state is
subject to small disturbances, so the precise equilibrium point is seldom
static. Instead, the entire dynamic optimization process occurs
continually in a proportional manner. This entire process is continually
repeated, utilizing proportional control, to ensure continuous operation
at the lean power loss/misfire limit of engine 12. Note, that the
optimization of this embodiment will occur on any engine, regardless of
the health of such engine, as the control point is a dynamic state, not
fixed to a given power point or definitive point in the engine operating
envelope (EOE).

EXAMPLE

Assume the governor set point GSP is 350 RPM, and k=1. Engine 12 in
accordance with the control established by system 10' is automatically
"leaned" until the engine speed is between 350 and 349 RPM (i.e., GSP-k;
350-1=349). In practice, the actual engine speed will fall somewhere
between the two set points. The foregoing provides full analog control,
and the tension between the two controllers (i.e., PID controller 58' ,
and the governor 24) will achieve a dynamic stability with infinite
resolution. The foregoing is not a dithering-type control. The lean power
loss/lean misfire limit in this example is thus defined as delta 1 RPM.
For various engine types, the value of k need not be a fixed constant, but
rather, can be optimized (not just for the engine and fuel) but over the
entire engine operating envelope comprising a range of engine speeds and
engine torques, among other engine operating parameters. At low power
settings, the k delta can be reduced because the drop off to the lean
power limit/misfire limit is more abrupt and the engine will likely be
more sensitive to air-fuel ratio changes. The control established by
system 10' is operable for all fuels, over the entire engine operating
envelope, and requires only the governor sensed variable as a control
input. Unlike control system 10, there is no need to sense torque in the
embodiment of system 10'. Control system 10' requires that engine 12 be
governed in some way (e.g., engine speed governed), that the default state
when no control is imposed is always at least slightly lean of
stoichiometric, and, that the governor 24 must have a faster response
characteristic than that configured for the controller 58'. Ideally, "k"
factor and the gain for the two controllers should be optimized over the
entire engine operating envelope (adaptive control).

In addition, as with the first embodiment system 10, fuel control valve 26
of system 10' is selected to fail in an unrestricted or open state (i.e.,
"on shelf condition"). In the event that computing device 28 fails, then,
the fuel control scheme is designed to fail fully open on a loss of
signal. Engine 12 would therefore nonetheless still be governed (i.e., by
governor 24), only the air-fuel ratio would not be optimized. Engine 12 is
left operable, and the operator can be notified by an alarm of this
occurrence. Alternatively, the same alarm can be used to stop the engine
operation if desired. This would be useful if engine 12 must be operated
at the emissions limits enabled by the control established by the present
invention. This failure mode is predicated on the fact that the air and
fuel delivery apparatus is selected by design to default to lean of
stoichiometric A/F operation, but not so lean as to be at the lean power
loss point/lean misfire limit. That is, as long as the default position is
anywhere on the lean side of stoichiometric A/F, it will work suitably
with the control of the invention.

System 10' does not require an active governor in all engine-operating
modes. At high engine power outputs, the governor may drive the air and
fuel delivery apparatus to 100% output (WOT) At that time, the power
control reverts to the fuel circuit alone. This control action transition
to and from governor 100% output is seamless, continuous, analog,
proportional, bumpless and accomplished without two state (on/off) sensors
and a "dithering" control action.

The entire process described in the foregoing paragraphs is repeated
continuously in an infinitely proportional analog process to keep the
engine air-fuel mixture optimized at all times. The above steps need only
be reversed to return the engine to its "as found" state.

Referring now to FIG. 13, in yet another embodiment, control system 10 and
control system 10' operate in accord with a unified hierarchy. In
particular, a steering logic portion 130 of this embodiment is configured
such that when both engine speed signal S.sub.5 and engine torque signal
S.sub.6 are available, the steering logic selects control system 10 to
control the air-fuel ratio of engine 12 in the manner set forth
hereinbefore. However, steering logic 130 is further configured such that
when the engine torque signal S.sub.6 becomes unavailable (either invalid
or absent or deselected by the operator or deselected by automatic control
logic), then steering logic 130 selects control system 10' to control the
air-fuel ratio provided to engine 12. In such an event, the steering logic
portion may be further configured to issue an alarm to the operator. The
operator then determines an appropriate response action according to
predetermined response procedures, which may be based on (i) emission
compliance goals, or, (ii) fuel efficiency. It should be appreciated that
when control system 10 controls the air-fuel ratio, both engine speed and
engine torque are known; therefore, emissions may be determined in accord
with known methods (as described hereinbefore). In this embodiment, the
air-fuel ratio provided to engine 12 is always optimized with respect to
fuel consumption, as well as emissions, such as CO.sub.2, NO.sub.x, CO,
CH.sub.2 O or other species of interest.

It is to be understood that the above description is merely exemplary
rather than limiting in nature, the invention being limited only by the
appended claims. Various modifications and changes may be made thereto by
one of ordinary skill in the art which will embody the principles of the
invention and fall within the spirit and scope thereof.